1. Field of the Invention
The invention relates to a mask for ion, electron or X-ray radiation beam lithography, which comprises a planar semiconductor wafer, and more particularly to such a mask in the form of a thin membrane. This invention also relates to a method of making such a mask.
Such masks are used to transfer the mask pattern (usually reduced by a factor of a 5) by means of projection irradiation with ions, electrons or X-rays into a photoresist layer.
2. Description of Related Art
A mask pattern deposited on a membrane is described in U.S. Pat. No. 3,742,230, and a mask pattern defined through a mask pattern generated in the membrane is described in European Patent No. 0 019 779 and corresponding U.S. Pat. No. 4,417,946 of Bohlen et al entitled "Method of Making Mask for Structuring Surface Areas". The methods for making such masks are described in the above-cited patents.
In the above European Patent the membrane carrying the mask is boron-doped. The effect of such boron doping is a high tensile stress in the thinned layer, with the consequence that the wafer is bent. In such cases where microminiaturized patterns of very high packing density are to be transferred with the mask, such bending cannot be tolerated. In the article "Shadow Projection Mask for X-Ray, Ion and Electron Beam Lithography" by J. Greschner et al., published in IBM Technical Disclosure Bulletin, Volume 26, No. 7A, December 1983, Page 3263, a solution for this problem is discussed which consists in that the 2-3 .mu.m thick boron doping is produced only in those regions of the semiconductor wafer which are subsequently thinned or where the mask pattern will be produced, respectively. Irrespective of whether the doped layer is a blanket coating over the one surface of the semiconductor wafer, or only over regions thereof, the thinned regions are tensioned owing to the tensile stress caused by the boron; and if noticeable warping of the semiconductor wafer is excluded, they are planar. Buckling of the membrane occurs only under the influence of a sufficiently high doping-dependent temperature. During the "Symposium on the Eighth International Conference on Electron and on Ion Beam Science and Technology" (1978) H. Bohlen et al lectured on the fact that upon a surface concentration of the boron of 10.sup.19 atoms/cm.sup.-3, the thin layer buckles at approximately 120.degree. C.
P. Nehmiz et al. "Electron Beam Proximity Printing: Complementary-Mask and Level-to-Level Overlay with High Accuracy" J. Vac. Sci. Technol. B, Vol. 3 (1) January/February 1985 (136-139) also published as the Proceedings of the 1984 International Symposium on Electron, Ion and Photon Beams, Tarrytown, N.Y., USA (1984), reported on mask distortion connected with the method used for making the masks. According to this method, the mask pattern is generated in or on the doped layer prior to thinning. As in this stage of the production, the doped layer is connected to the remaining substrate over its entire width, which prevents effects of the tensile stress, the transferred pattern is a true image of the desired pattern. If there is subsequent selective thinning, the tensile stress in the membrane which is now fixed only at its periphery causes a distortion of the pattern in a mask plate. This so-called "cold distortion" equals zero in the center and in the periphery of the membrane, and it is at a maximum in a zone being closer to the center than to the periphery. The "cold distortion" can be compensated by a corresponding doping with a material generating pressure tension in the silicon. The quantity of doping material required for compensating the "cold distortion" depends on the temperature. To give an example: If at a temperature of 40.degree. C. the membrane is to be tension-free and thus undistorted, the quantity required is lower than if the membrane is to be tension-free at 20.degree. C. An undistorted and planar pattern however is achieved only if this predetermined temperature can be maintained over the entire membrane. However, if the masks are to be used in accordance with their purpose, i.e. in the irradiation with ion, electron and X-rays, it is not possible to achieve a uniform temperature over the thinned layer. In fact, the membrane will always be hotter in its center than in its periphery. The uncompensatable distortion increases with the temperature difference between the periphery and the center.
In electron beam exposure it is known to keep the maximum beam current at such a low level (1.gtoreq.10 microamperes), that the temperature increases T are not higher than approximately 10.degree. C.; and in X-ray lithography, mask heating during exposure is prevented by a helium atmosphere in the environment which ensures exhausting of the heat generated in the mask. This helium atmosphere has some disadvantages. On the one hand, owing to the absorption of X-rays it limits the spectrum available for lithography. On the other hand, it involves considerable (safety) technical efforts since by means of several pressure stages the helium pressure has to be successively adapted to the high vacuum at the site of the X-ray source. During operation, extremely high-speed valves must assure that should a window burst there will be no gas breaking-in at the site of the source.